Folate is the essential vitamin sometimes referred to as Vitamin B9, but more commonly known by its synthetic supplemental form 'folic acid'. Found in low levels across most food products of plant origin and found in enhanced levels in some nation's wheat grain due to fortification, folic acid is a vitamin that has essential roles in the growth of neonates as well as in supporting a process known as methylation in adults.
Folic acid is primarily known as being the 'pregnancy supplement' where women who are planning to conceive children take 400 μg folic acid daily. This is critical in preventing something known as 'neural tube defects' (NTDs) which are caused by insufficient folate provision to the fetus when their neural tube is being formed and the combination of food fortification and supplementation has greatly prevented.
Beyond that, folate and folic acid are used to support methylation in the body. These two supplements, due to eventually forming a molecule known as 5-methyltetrahydrofolate (5-MTHF or L-methylfolate) are involved in indirectly supporting and creating S-Adenosyl Methionine (SAMe) in the body. Due to this, these supplements tend to show quite a bit of similarity to SAMe supplementation and some parallel to supplements involved in the alternate pathway which support SAMe levels (of which supplements like creatine and trimethylglycine (TMG) are implicated in). These parallels are best demonstrated in their effects on depression where they all seem to be implicated in supporting serotonin-based medication (SSRIs usually) as adjuvant with a trend to benefit women more than men.
Despite these benefits, folic acid is unlike other B-vitamins where it does appear to have a potential side-effect when taken in high doses. While taking a very large dose of folic acid several thousand times the RDA will not cause immediate harm it appears that prolonged exposure to levels up to 250% the RDA may be associated with a relative increase in the rates of cancer, particularly colon cancer among the elderly; well demonstrated by acute increases in the rates of colon cancer when both Canada and the US introduced fortification (the rates, of which, have been declining beforehand and ever since). Among the B-vitamins, folate seems to be the one where a balance should be strived in your everyday life and high sources of supplemental folate should be avoided if not intended for other purposes.
Finally, supplements tend to be in one of three forms; folate, folic acid, and L-methylfolate. Of these supplements folic acid is the most common but is thought to be negative when overconsumed. L-methylfolate seems the most promising as many people seem to have genetic mutations in the enzyme which produces L-methylfolate from folate and folic acid, so supplementing the latter two only causes a partial backlog in these people whereas supplementing L-methylfolate may circumvent this genetically disadvantageous rate-limiting step. Furthermore, L-methylfolate seems to be the most promising agent to support SAMe levels as SAMe, as a supplement, is quite expensive and this pathway tends to take on more of the workload than the one creatine and TMG are implicated in.